Rheumatoid factor and cryoglobulinemia in chronic viral infection Mixed cryoglobulinemia (MC) is strongly associated with chronic HCV infection. MC is an immune complex-mediated medical condition and is found in 40-60% of chronic HCV patients, with 5-15% patients develop clinical symptoms ranging from purpura, arthralgia, skin ulcers, peripheral neuropathy, glomerulonephritis and myocarditis. HCV-related MC greatly affects patients? quality of life therefore a priority for HCV treatment. The pathogenic mechanism of MC is poorly understood, although it is suggested that HCV infection can stimulate unregulated B cell proliferation, and in some cases, the production of anti-antibody, i.e. rheumatoid factor (RF), of the IgM isotype. The HCV-induced RF in turn forms immune complexes with anti-HCV antibodies and HCV antigens that are sensitive to precipitation at temperature below 37oC which are known as cryoglobulins. However, recent studies reported that successful HCV treatment can improve symptoms of MC but cryoglobulins may still persist, indicating other non-viral antigens may be involved in the formation of cryoglobulins. In Aim 1 of this proposed study, we will identify the antigens present in cryoglobulins by a proteomic approach, both before and after successful HCV treatment, in order to decipher the composition and diversity of cryoglobulins and the mechanism of their formation. In Aim 2, we will study RF from MC patients in order to understand the diversity of antibody gene usage and their target epitopes, and whether HCV clearance may restore RF to normal level. Our goal is to develop novel experimental systems and reagents in this exploratory/developmental project that will be useful for understanding the disease mechanism of MC, and for defining the molecular properties of RF in different microbial infections and autoimmune conditions in the long term.